Do you mean something like an ipad which is just a screen and no keyboard? I prefer to use a real keyboard/full-size screen when planning to do any sort of typing such as composing an email/etc. Laptops are available in various screen sizes. While you can surf the internet on your smartphone, the tiny screen often jumbles the display and has issues with word-wrap since many sites seem formatted for best viewing on a full-size screen.
As with anything computer related, it tends to depend on what your preferences are, what your budget is and how exactly you plan to use it.